text: While all deactivating groups are inductively withdrawing (–I), most of them are also withdrawing through resonance (–M) as well.  Halogen substituents are an exception: they are resonance donors (+M).  With the exception of the halides, they are meta directing groups.
  Halides are ortho, para directing groups but unlike most ortho, para directors, halides mildly deactivate the arene. This unusual behavior can be explained by two properties:
  # Since the halogens are very electronegative they cause inductive withdrawal (withdrawal of electrons from the carbon atom of benzene).
  # Since the halogens have non-bonding electrons they can donate electron density through pi bonding (resonance donation).
  The inductive and resonance properties compete with each other but the resonance effect dominates for purposes of directing the sites of reactivity. For nitration, for example, fluorine directs strongly to the para position because the ortho position is inductively deactivated (86% para, 13% ortho, 0.6% meta).  On the other hand, iodine directs to ortho and para positions comparably (54% para and 45% ortho, 1.3% meta).